The present invention relates to improving images produced by utilizing digital halftones or spatial dithering in electrostatographic printers and reprographic copiers and more particularly, concerns solving the image problems of edge delineation, leading edge deletion, grayscale and highlight fidelity in an image. Such leading edge deletion and placement problems may manifest as phenomena often referred to as line shrinkage, halo and white gap artifacts. These artifacts are also sometimes referred to as “slow toner”. However, when halftone dots are employed, the more apparent manifestation of the above slow toner problem includes the loss of small highlight dots and shadow detail which may not print at all, and other halftone dots that may print only in part. The result is a reduction in printer tone gamut as well as distortion of the printer Tonal Response Curve (TRC).
In a typical electrophotographic printing machine, a photoreceptor is charged and exposed to record an electrostatic latent image therein. The photoreceptor is advanced to a development station where toner is deposited on the latent image.
One type of development system forms a toner cloud in the development zone. One type of development system in which a toner powder cloud is described is U.S. Pat. No. 4,868,600, which is herein incorporated in its entirety by reference.
It has been found that when a toner powder cloud development system is used, large amounts of toner are deposited where there is uninterrupted expanse of charged area (such as toward the middle of large image shapes), and toner is starved from locations where there is a strong or sudden change in charge (as found with narrow lines or shapes and on the edges of larger shapes). The result for digital halftones is a loss of highlight and shadow detail. The effect on large halftoned shapes causes them to exhibit a defect called halo, which manifests itself most clearly at the interfaces of solid colors. Halo in color systems appears as white lines at interfaces which should otherwise be a perfect match between two colors. This defect is also observable in single color images as an edge distortion or displacement and line shrinkage dependent on the size of the printed object. Line shrinkage of course leads to poor line and text quality due to an erosion or shrinkage of the line edges and corners. Various approaches have been devised to solve these highlight and shadow detail slow toner effects and thus extend the TRC and printer tone gamut.
The following patents appear to be relevant to manipulation and enhancement of the edge of image shapes.
U.S. Pat. No. 5,029,108 to Lung discloses an edge enhancement method and apparatus for dot matrix devices wherein a group of gradient mask matrices are applied to a “current matrix”, wherein a target pixel is surrounded by neighboring pixels, to determine if the target pixel is at a location where a change of brightness occurs. From this matrix operation, a conclusion is derived as to the existence or non-existence of an edge and the direction of the brightness change. The current matrix and a predetermined number of previously evaluated and yet to be evaluated pixels are then compared to a set of reference bit patterns which depict possible segment changes to be corrected. If the result indicates that the target pixel is on an edge of a changing edge segment, a corresponding code will be generated to modify the target pixel to enhance the smoothness of a segment transition. In the case of an electrophotographic printing machine, the specific code will change either the location or the size of the target pixel; whereas in the case of a monochrome screen display, the specific code will change the intensity of the target pixel.
The U.S. Pat. No. 5,479,175 to Cianciosi et al. is a an apparatus for enhancing the output along edges of discharged area developed regions in a tri-level imaging system employing a pulse width and position modulated signal ROS for exposure. The invention enables the identification and selective alteration of video data used to drive the ROS so as to extend the developed regions by a selected amount and eliminate digitization artifacts present in the image to be printed. The extension of the discharged area developed regions is accomplished by extending the width of, or adding separate, exposure pulses in adjacent areas to enable development within a portion of those regions.
In U.S. Pat. No. 5,193,008 to Frazier et al., the output of a conventional laser printer having a resolution of 300×300 dots per inch (dpi), and a predetermined threshold level for forming image dots, is enhanced by selectively providing interleaved image dots between the normal scan lines of the laser printer. Such interleaved image dots between scan lines may be achieved by appropriately energizing the two pixels directly above and directly below that desired interleaved dot, with the energizations at one or both pixels being selectively below the threshold level for producing a dot on the scan line, but with the combined energization at the desired interleaved point being above the threshold level to produce the desired interleaved dot. An input 600×600 bit map may be stored in a random access memory, and three vertically aligned bits from one main scan line and adjacent 600 dpi lines above and below are drawn from the RAM and are supplied to a logic and video output circuit which produces variable pulse width modulated pulses to the laser printer to produce the enhanced image.
U.S. Pat. No. 3,784,397 discloses a method for forming images by providing an electrostatographic imaging member bearing an electrostatic latent image on a recording surface. Then positioning the recording surface spaced from and facing a development electrode. This is followed with contacting the recording surface with toner particles whereby at least a portion of the toner particles deposit on the recording surface to form at least a partially imaged recording surface. Then maintaining the field strength of the development electrode as weak during the initial period of development and then increasing the field strength of the development electrode during the latter period of development, to form a substantially uniform developed image substantially free of streak, halo, edge effect, and background deposits.